This invention relates to thrust bearing assemblies that typically comprise a retainer ring encompassing a plurality of thrust bearing pads arranged in circumferential spaced relationship around a shaft axis. A shaft that is to be axially supported for rotation usually has a radially extending collar providing a bearing surface that interfaces with the array of thrust pads which are bathed in lubricant. The thrust pads are customarily supported on a circular array of overlapping pivotal rockers or leveling links that transmit forces between each other to equalize the load on the thrust pads. The pads are free to tilt to a limited extent so as to develop a wedge-shaped film of lubricant that supports the load. The load on each bearing pad is inversely proportional to the oil film thickness squared. To equalize the load, the leveling links lower the overloaded thrust pad and raise the underloaded thrust pad. Since the leveling links are in contact, any unbalanced forces are transmitted through the leveling links until equilibrium is reached.
Under actual operating conditions the thrust pads in existing bearing designs are not loaded perfectly evenly even though, theoretically, their loads should be equalized due to the force distributing interactions between the leveling links. Applicant has determined that incomplete equalization is due primarily to friction between contacting areas of the adjacent leveling links. These frictional forces are generally displaced from the pivot points of the leveling links and, hence, the frictional forces act through a moment arm to resist free pivoting of the leveling links that is required for complete load equalization among the thrust pads.
Another characteristic of prior art tilting pad thrust bearings is that the surfaces of the various thrust pads on which the shaft collar runs become slightly convex or crowned in the radial and circumferential directions during actual operation. This is caused by the unequal thermal expansion between the front side of the thrust pad facing the collar where the frictional heat is developed and the back side which is cooler and also by the effect of the distributed load on the front side and a point contact support on the back side. Some convexity or crowning in the circumferential direction is beneficial since it assists in generation of the lubricant wedge. Any radial crowning is detrimental, however. It reduces the hydrodynamic forces inherent in the lubricant wedge and tends to cause breakdown or thinning of the wedge near the center of the thrust pad.